This invention relates generally to fixtures for positioning workpieces on machine tools and more particularly to a fixture which is equipped with a pneumatic system for accurately positioning workpieces on a machine tool table.
Pending application Ser. No. 852,565, filed Apr. 16, 1986 by Lane et al. and owned by the assignee of the present application, discloses a fixture system which is specially constructed to be set up by robotic machines. The work table of the machine tool is provided with openings arranged in a grid pattern in order to receive removable locator pins that act as stops against which workpieces can be positioned. Because the openings have known locations, the location of a workpiece placed against the pins inserted in the openings is known. Consequently, machine tool operations such as drilling and milling can be carried out accurately by the tool head. The system disclosed in the aforementioned application is especially well suited for robotic machines because the openings and pins are tapered and the pins do not have to be precisely aligned with the openings in order to be inserted into them. Accordingly, the inability of robotic machines to precisely align the pins with the openings is not a severe handicap since all that is required is for the alignment to be close enough to allow the tapered portions of the pins to seat themselves in the tapered openings.
Special measures are required to enable the fixture system described above to be useful in the machining of different parts. For example, parts that are unusual or irregularly shaped must be held closely between pins which are inserted in the openings in the work table. However, if the dimensions of the part are such that the part does not exactly span the openings, then the simple locator pins are not able to securely hold the part in a fixed position. The present invention is directed to a fixture system in which all parts can be securely held in a fixed and known position on the work table.
In accordance with the invention, some of the sides of a workpiece are positioned against locator pins which are fitted in selected openings in the work table. Other openings receive specially constructed pegs that are equipped with pneumatic cylinders. The ends of the cylinder rods carry guide blocks which can be extended against the workpiece in order to maintain it against the locator pins such that the workpiece is held in a known position while drilling, milling or other machining operations are carried out. These special pegs have conical bases so that robotic machines can insert them into selected openings in the table without first aligning them precisely with the openings.
It is a particular feature of the invention that air for operating the pneumatic cylinders is made available at each opening and can be selectively applied only to those openings in which a peg is seated. Air passageways are formed through the table, and each passageway leads to one of the openings in the table. Pneumatic lines which supply the individual passageways are equipped with valves which permit the air flow pattern to be adjusted to apply air only to the openings which receive a peg. Each opening has an annular groove which receives the air supplied to the opening.
Each peg has an air passage which directs air to the cylinders from the groove of the opening in which the peg is received. Diametrically opposed ports in the conical base of the peg form air inlets which admit air to the passage in the peg, and the ports register with the annular groove regardless of the rotative orientation of the peg in the opening. Consequently, each peg that is fitted in an opening receives air for operation of its cylinders whenever the corresponding valve is open. The application of air to the cylinders results in extension of the guide block against the workpiece, and the air pressure holds the workpiece firmly in place against the locator pins during machining operations.